Tank closure devices, such as have been disclosed by DE 198 37 783 A1, possess a housing which is penetrated by an opening for the filling of a tank, whereby the opening can be closed internally by a shut-off flap, which is spring loaded in its closing direction. On the closure flap is to be found, in the known closure device, a sealing ring serving as an O-ring, which coacts with a sealing-seat, which seat is conical and widens toward the inside of the tank. In order to achieve a high degree of sealed off tightness (for example LEV-II or PZEV-Norm), the sealing ring must be pressed against the sealing seat at a relatively high pressure. In a multitude of cases, the conventional tank closure devices accordingly operate with strong springs. It then becomes quite possible, that a greater expenditure of force is required of the user for the opening of the closure flap, and that, from the standpoint of evaporation measures, it is required to avoid, on the snapping shut of the closure flap, a wide spread spraying of fuel or indeed even a danger of traumatic personal harm. In the case of other known modes of fuel tank closure design, somewhat similar to DE 198 37 783 A1, weak springs were allowed, which permitted an easy opening of the closure flaps. In this case, however, it is still expensive in time and effort, to bring the sealing ring against the locking mechanism which presses against sealing seat, in order to achieve the necessary pressure for sufficient sealing action.